Binding of hydrophobic peptides to several non-catalytic sites promotes peptide hydrolysis by all active sites of 20 S proteasomes -: Evidence for peptide-induced channel opening in the α-rings

The eukaryotic 20 S proteasome contains the following 6 active sites: 2 chymotrypsin-like, 2 trypsin-like, and 2 caspase-like. We previously showed that hydrophobic peptide substrates of the chymotrypsin-like sites allosterically stimulate peptide hydrolysis by the caspase-like sites and their own cleavage. More thorough analysis revealed that these peptides also stimulate peptide hydrolysis by the trypsin-like site. This general activation by hydrophobic peptides occurred even if the chymotrypsin-like sites were occupied by a covalent inhibitor and was highly cooperative, with an average Hill coefficient of 7. Therefore, this stimulation of peptide hydrolysis at all active sites occurs upon binding of hydrophobic peptides to several non-catalytic sites. The stimulation by hydrophobic peptides was not observed in the yeast DeltaNalpha3 mutant 20 S proteasomes, in 20 S-PA26 complexes, or SDS-activated proteasomes and was significantly lower in 26 S proteasomes, all of which appear to have the gated channel in the a-rings in an open conformation and hydrolyze peptides at much faster rates than 20 S proteasomes. Also the hydrophobic peptides altered K-m, V-max. of active sites in a similar fashion as PA26 and the DeltaNalpha3 mutation. The activation by hydrophobic peptides was decreased in K+-containing buffer, which favors the closed state of the channels. Therefore, hydrophobic peptides stimulate peptide hydrolysis most likely by promoting the opening of the channels in the a-rings. During protein breakdown, this peptide-induced channel opening may function to facilitate the release of products from the proteasome.